Like Julian I found myself reading Archaeology at uni and this was one of the first things we learned.
I was taught by the late Mr May MA BA (Oxon)
He was a brilliant linguist who had served his National Service in the RAF at Gatow. He spoke Russian, Czech etc.
The RAF Regiment on the airfield perimeter had to hold the Russians for 5 minutes so they could destroy all the papers and machines in the listening rooms.
Time after which
Time before which
As did I (reading archaeology that is) and I have been working in the "field" for 25 years and still am.